Monodentate, definition

Attention should be paid to the fact that the ratio of Pd and phosphine ligand in active catalysts is crucial for determining the reaction paths. It is believed that dba is displaced completely with phosphines when Pd2(dba)3 is mixed with phosphines in solution. However the displacement is not eom-plcte[16]. Also, it should be considered that dba itself is a monodentate alkene ligand, and it may inhibit the coordination of a sterically hindered olefinic bond in substrates. In such a case, no reaction takes place, and it is recommended to prepare Pd(0) catalysts by the reaction of Pd(OAc)2 with a definite amount of phosphinesflO]. In this way a coordinatively unsaturated Pd(0) catalyst can be generated. Preparation of Pd3(tbaa)3 tbaa == tribenzylidene-acetylacetone) was reported[17], but the complex actually obtained was Pd(dba)2[l8],... 

With these definitions, it is possible to classify all monodentate ligands into one of the following five types ... 

This type includes the common examples of the 0,0-bidentate keto form (I) and less common O-monodentate enol form (II) and r/2-C2-bondcd enol form (III). In the absence of definitive X-ray data, the latter was carefully investigated by IR and NMR techniques because of the possibility of the additional two structures (Illb) and (IIIc). 

This leads to unique definition of the stability constants of monodentate surface complexes involved in the specific adsorption of cations. Thus, reaction (5.65) rather than reactions (5.66)-(5.68) should be chosen as a standard definition, according to the above standards. The exact definition of the stability constant of the =TiOCd species in the present example depends on the electrostatic position of Cd in the surface complex. It should be also emphasized that even with fixed electrostatic position of Cd, the numerical value of the equilibrium constant of reaction (5.65) depends on the choice of the model of primary surface charging. The details on the models of primary surface charging are not given in the tables in Chapter 4, and the reader is referred to the original publications. 

The above discussion was focussed on monodentate surface species. With bi-and tridentate surface species another controversy should be addressed, with respect to definition of stability constants of a surface complex (involving anion or cation). Let us consider the following surface reaction ... 

Definition The ring chemical shift is dehned as the valne observed for the metallacycle compared to the chemical shift fonnd for a complex where the chelate ligand is replaced by two eqnivalent monodentate ligands. 

Among the current spectroscopic methods extended x-ray absorption fine structure spectroscopy (EXAFS) has received the most attention, however other methods such as FTIR, may be equally as promising. EXAFS is considered to provide definitive information on inner- vs. outer-sphere bonding and is suitable for determining mode of attachment to the surface (monodentate, bidentate, binuclear) but does not resolve questions of surface speciation since it is not sensitive to H atoms. In addition, examination of the same system by different researchers has in some instances resulted in different conclusions. 

Earlier, Tejedor-Tejedor and Anderson (5) interpreted C2V and 3 symmetries of adsorbed phosphate as bidentate bridging complexes. These authors indicted that some of their stretching modes are also compatible with a monodentate complex. Controversy in interpretation of FTIR spectra of adsorbed phosphate, as with interpretation of Se XAFS results (discussed below) suggests that spectroscopic results by themselves, are not always definitive. 

Monodentate selenito complexes can be distinguished from bidentate complexes by examining the V3 splitting discussed above. Monodentate complexes split into a doublet while bidentate complexes split into two distinct bands (26). Inspection of the peaks in Figure 7 suggests that they are separate peaks consistent with bidentate complexation. This is not definitive since the separation of the peaks (161 cm ) is closer to the separation seen for the monodentate bands than for that reported for the bidentate selenito complex (Table I). 

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